The structural characteristics of the human lumbar spine have been studied for many years. However, experimental data has not identified the nonlinear, coupled, viscoelastic stiffness properties of the intervertebral joints, and individual component contributions are mostly unknown. The objectives of this biomechanics program are: to experimentally determine the stiffness characteristics of the lumbar intervertebral joints over a wide range of multiple loadings; to determine the role of individual joint components in the overall joint stiffness; to establish the response of individual components in external loading; to quantify the viscoelastic properties of the joint; and to develop an analytical model, consistent with the above data, to predict the load displacement response of a joint and the strain in joint components. Experiments will be based on the stiffness method whereby fixed displacements are applied across a joint while all of the resulting forces and moments are measured. Initial displacements in the remaining five directions will be established to determine the effect of one or more displacements on the load-deflection characteristics in another direction. An MTS electro-hydraulic load frame will be used to activate the displacement devices. A computer will be used to control the servo-activator, and to digitize and reduce the data. The analytical model of the isolated lumbar spine will be a three dimensional lumped parameter model including material and geometric nonlinearities.